webadmin: Failed to add VM network interface from User Portal

ConfigurationValues.ManagementNetwork doesn't have ConfigAuthType.User.
It means user role can't determine this value.

The user shouldn't know what is the management network.
If the management network can't be determined the default network should be
the first network in the list.

Change-Id: I77a15bd0d9d290c8f7e2673f0e0039d70f125c1b
Bug-Url: https://bugzilla.redhat.com/912005
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>